I have a nail in my tyre, should I just leave it in because every time I take it out it goes more flat?

Take the wheel/tire OFF of the car. 

Install the SPARE. 

Bring the leaking tire to a tire shop and get it checked, patched and high speed balanced. 

Reinstall it on your car. 

If you drive with too little air pressure your tire will overheat and may rip apart.  Or almost rip apart. 

Just get it repaired!! 

Nail is filling the hole right now so leave it.  Better than having a hole spurting out air.  How is it you find nails and I find screw driver bits= much bigger hole= instant flat.2 times 2 of the same model but different cars half way around the world on a dirt road in the middle of nowhere both times.?  Yeah, you put on the spare tire. Many people don't carry a spare and holes happen ANYWHERE ANYTIME.

Leave the nail in place and drive to a tire shop to get the tyre patched. 

No Dufus...go get the tire patched!

Visit a tyre fitters asap with the nail in the tyre. A repair is cheap. If you keep driving on it it could deflate suddenly at any time. No fun on a motorway.

You have a hole in your tyre

GET IT FIXED.

Keeping the nail in is a TEMPORARY fix and over time will make the hole bigger and harder to repair.